The present invention relates to a sync (synchronization) signal detecting device for detecting a sync signal included in a digital signal.
In transmitting or recording a digital signal, in order for the digital signal to be readily reproduced, in general, a sync signal is arranged at a predetermined position in each frame of digital data; various other data comes after the sync signal.
A conventional sync signal detecting device of this type, as shown in FIG. 1, includes a shift register 1, a sync signal pattern generating circuit 2, and a comparison circuit 3 for comparing the output signal of the shift register 1 with the output signal of the sync signal pattern generator 2.
A digital data signal applied to the shift register 1, which is transferred in synchronization with a clock signal data signal is equal in length to the sync signal included in the digital data signal. The sync signal pattern generating circuit 2 generates the same pattern as that of the sync signal in synchronization with the clock signal. In the comparison circuit 3, the output signal of the shift register 1 is compared with that of the sync signal pattern generating circuit, and when they are the same, a detection signal is produced.
However, since the conventional sync signal detecting device provides the detection signal only when the input signal and the sync signal pattern coincide completely with each other, if the sync signal cannot be detected, the data of the frame cannot be read.
In order to overcome this difficulty, heretofore the sync detecting device has been provided with a sync signal protecting device which operates in accordance with the expected period of arrival of the sync signal so that when the sync signal cannot be detected within a predetermined period, a "hunting" operation is performed or a false sync signal is outputted. However, the device is necessarily intricate in construction and high in manufacturing cost.
Even if such a sync signal protecting circuit is added to the detecting device as described above, it is difficult to positively detect the sync signal in the case where it is not always periodically produced, for instance, in the case of an optical audio disc EFM signal recorded on a video disc. (See Japanese Patent Applications Nos. 27250/1985 through 272255/1985 filed by the present applicant.)